In short-distance optical interconnects applications, low energy consumption as well as high transmission speed of the building block devices is becoming a key technological issue as the data transmission bandwidth increases. Thus, the figure of merit is energy consumption per transmitted bit. According to a recent technology roadmap provided in [1], a few 10 s fJ/bit is required in 2015-2020 for light transmitters of chip-level optical interconnects.
As a light emitter, vertical-cavity surface-emitting lasers (VCSELs) are one of preferable existing solutions. It is because their fabrication technology is matured and their energy consumption is much smaller than that of edge-emitting lasers due to their small active material volume. To send a bit signal, output light intensity of a light emitter should be modulated. There are two ways of modulating the output light intensity; direct modulation and indirect (or external) modulation. Among these two approaches, direct modulation is easier to implement since external modulation approach needs an external modulator.
In the direct modulation scheme, the current injection to a laser is modulated. This leads to the intensity modulation of the output light. A state-of-the-art result is reported in [2]. The transmission speed was 35 Gb/s, the energy consumption excluding the RF driver circuitry was 12.5 mW, and the emission wavelength was 980 nm. The demonstrated energy per bit of 357 fJ/bit (=12.5 mW/35 Gb/s) is remarkably small but is not sufficient for the aforementioned applications. The weakness of this approach is that it is difficult to further increase the speed or reduce the energy consumption: Speed of a laser diode is decided by its intrinsic response and circuit response. The intrinsic speed is defined by −3 dB bandwidth of the intrinsic frequency response which is proportional to relaxation oscillation frequency, fr:
                              f          r                ∝                                            I              -                              I                th                                                    V              p                                                          (        1        )            where I is the injection current, Ith, threshold current, and Vp, modal volume. In order to obtain higher intrinsic speeds, the injection current needs to be higher while the modal volume, preferably smaller. In the demonstrated VCSEL, the modal volume is not likely to be further reduced. It is because its transverse mode size and effective cavity length that determine the modal volume are difficult to be further reduced. In the demonstrated VCSEL, the oxide aperture diameter of 3 μm is already the smallest with a reasonable optical loss. If one decreases the oxide aperture size below 3 μm to obtain a smaller modal volume, the optical loss dramatically increases, leading to higher Ith. In Eq. (1), a higher Ith decreases the speed. Regarding the injection current, if one increases the current for higher intrinsic speed, it will result in higher energy consumption. On the other hand, if one decreases the current for smaller energy consumption, it will result in slower intrinsic speed. Thus, it is difficult to further increase the speed and decrease the energy consumption simultaneously, based on the conventional VCSEL structure. One should also consider that high injection current is detrimental to long-time stability of small-volume lasers. The speed related to the circuit response is mainly decided by the series resistance and capacitance of the laser structure. In the demonstrated VCSELs, these parasitic terms were already tightly suppressed. Thus, a significant improvement in speed related to parasitic terms is not expected.
As discussed above, the transverse mode size of 3 μm is already the smallest with a reasonable optical loss. In VCSELs, the effective cavity length is the sum of a nominal cavity length and field penetration into distributed Bragg reflectors (DBRs). Since one needs an optical cavity with a certain thickness that includes an active region for light generation, one cannot significantly reduce the effective cavity length, either.
Thus, there is a limit on the speed and energy consumption of directly-modulated VCSEL structures, in currently know solutions. For further improvement beyond the state-of-the-art energy per bit values, one needs an innovative laser structure.